diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4e79d60..d7dacd3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -81,6 +81,10 @@ jobs:
         env:
           DC_SANDBOX_NET_TEST: '1'
           DC_REQUIRE_RIPGREP: '1'
+          # Linux is the only runner with bubblewrap, and the bwrap integration
+          # tests are the only ones that check what the sandbox does rather than
+          # what arguments it builds. Make a missing binary fail there.
+          DC_REQUIRE_BWRAP: ${{ runner.os == 'Linux' && '1' || '0' }}
         run: pnpm test
 
       - name: Build + app-server release gate
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2ff7e2a..9209054 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -48,6 +48,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ask, and a hit is not yet a read.
 - The plugin capability bridge passed no contract into the tools it executed, so
   a plugin's `Grep` skipped the same filter.
+- **`--sandbox read-only` was not read-only on Linux.** `buildLinuxBwrapArgs`
+  ended with an unconditional `--bind <cwd> <cwd>`, and bwrap applies binds in
+  order with the last one winning — so the read-only bind that the mode had
+  correctly asked for was overwritten a few arguments later, and a command could
+  write to the workspace. macOS never had this: its profile grants writes only
+  from `allowWrite`, which read-only leaves empty. #226 introduced the mode axis
+  and verified it on macOS; this is the half nobody looked at. Callers using the
+  legacy `enabled: true` shape are unaffected.
 
 ### 🐛 Fixed
 
diff --git a/docs/THREE_WAY_REVIEW.md b/docs/THREE_WAY_REVIEW.md
index 985033c..980e308 100644
--- a/docs/THREE_WAY_REVIEW.md
+++ b/docs/THREE_WAY_REVIEW.md
@@ -293,7 +293,13 @@ AskUserQuestion、模式/模型/effort 下拉、Inspector、FilePanel（Source/D
   协议了，但 `Sidebar.tsx` 绕过 shim 直接 `listSessions()`——所以"第二个读取者"其实一直是三处
   而不是两处。delete 协议侧原本没有方法，新增 `thread/delete`：app-server 是 thread 存储的
   单一 owner，renderer 越过它删文件可能把正在写的 writer 的地板抽掉。
-- **Linux (bwrap) 侧只共享了 sandbox 模式解析，没有在 Linux 主机上做过 #226 那样的实测。**
+- ~~**Linux (bwrap) 侧只共享了 sandbox 模式解析，没有在 Linux 主机上做过 #226 那样的实测。**~~
+  已补：bwrap 集成测试新增 mode 轴（read-only 可读不可写 / workspace-write 可读可写 /
+  danger-full-access 不包 bwrap），Linux CI 用 `DC_REQUIRE_BWRAP=1` 禁止静默跳过。
+  实测立刻抓到一个真 bug：`buildLinuxBwrapArgs` 末尾无条件 `--bind cwd cwd`，
+  bwrap 后绑定覆盖先绑定，所以 **`--sandbox read-only` 在 Linux 上根本不只读**。
+  macOS 侧没有这个问题（写权限只来自 `allowWrite`）—— 正是"只在一个平台上实测过"
+  的代价。
 - 图片输入仍是空壳（有意保留：DeepSeek 无 vision 模型）。
 
 ## 附：Codex 侧信息的可信度声明
diff --git a/packages/core/src/sandbox/bwrap-integration.test.ts b/packages/core/src/sandbox/bwrap-integration.test.ts
index 0505773..7d0ba5f 100644
--- a/packages/core/src/sandbox/bwrap-integration.test.ts
+++ b/packages/core/src/sandbox/bwrap-integration.test.ts
@@ -6,11 +6,11 @@
 // Spec: docs/DEVELOPMENT_PLAN.md §3.9a
 
 import { execSync, spawn } from 'node:child_process';
-import { mkdtemp, readFile, rm } from 'node:fs/promises';
+import { mkdtemp, readFile, rm, writeFile } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { afterEach, beforeEach, describe, expect, it } from 'vitest';
-import type { SandboxConfig } from '../config/types.js';
+import type { SandboxConfig, SandboxMode } from '../config/types.js';
 import { wrapBashCommand } from './index.js';
 
 function hasBwrap(): boolean {
@@ -18,6 +18,13 @@ function hasBwrap(): boolean {
     execSync('command -v bwrap', { stdio: 'ignore' });
     return true;
   } catch {
+    // These are the only tests that observe what the Linux sandbox *does*
+    // rather than what arguments it produces, and they self-skip. On the Linux
+    // runner that has to be a failure: a green suite that skipped every real
+    // enforcement check is how `--sandbox read-only` stayed writable.
+    if (process.env.DC_REQUIRE_BWRAP === '1') {
+      throw new Error('DC_REQUIRE_BWRAP=1 but bwrap is not on PATH');
+    }
     return false;
   }
 }
@@ -32,8 +39,9 @@ async function runSandboxed(
   userCommand: string,
   cwd: string,
   config: SandboxConfig,
+  defaultMode?: SandboxMode,
 ): Promise<RunResult> {
-  const wrapped = await wrapBashCommand({ userCommand, cwd, config });
+  const wrapped = await wrapBashCommand({ userCommand, cwd, config, defaultMode });
   return new Promise<RunResult>((resolve) => {
     const child = spawn(wrapped.command, wrapped.args, { cwd });
     let stdout = '';
@@ -51,6 +59,8 @@ describe.skipIf(!RUN)('bwrap sandbox (real-kernel integration)', () => {
   let cwd: string;
   beforeEach(async () => {
     cwd = await mkdtemp(join(tmpdir(), 'dc-bwrap-int-'));
+    // Written outside the sandbox, so a read-only run has something to read.
+    await writeFile(join(cwd, 'seeded.txt'), 'from outside\n');
   });
   afterEach(async () => {
     await rm(cwd, { recursive: true, force: true });
@@ -87,4 +97,86 @@ describe.skipIf(!RUN)('bwrap sandbox (real-kernel integration)', () => {
     );
     expect(r.stdout).not.toContain('exit=0');
   }, 20_000);
+
+  // The `--sandbox` axis added in #226 was live-verified on macOS only, where it
+  // exposed a profile that denied reads of the project directory itself. Linux
+  // shared the mode *resolution* and nothing more: `sandboxConfigForMode` was
+  // unit-tested, `buildLinuxBwrapArgs` was argument-tested, and no test ever
+  // spawned bwrap in a named mode to see what a command could actually do.
+  //
+  // A read-only sandbox that cannot read is the failure macOS had. A read-only
+  // sandbox that can write is the failure worth catching here.
+  describe('modes', () => {
+    // No `mode` and no `enabled`, so the mode comes from `defaultMode` — the
+    // path every host takes, since #226 made workspace-write the default rather
+    // than something each caller sets.
+    const unset: SandboxConfig = {};
+
+    it('read-only: the workspace is readable', async () => {
+      // This is the assertion macOS failed before #226: a sandbox that denies
+      // reads of the project directory makes every command useless.
+      const r = await runSandboxed(`cat ${cwd}/seeded.txt`, cwd, { ...unset, mode: 'read-only' });
+      expect(r.code).toBe(0);
+      expect(r.stdout).toContain('from outside');
+    });
+
+    it('read-only: the workspace is not writable', async () => {
+      const r = await runSandboxed(`echo nope > ${cwd}/written.txt`, cwd, {
+        ...unset,
+        mode: 'read-only',
+      });
+      expect(r.code).not.toBe(0);
+      expect(r.stderr.toLowerCase()).toMatch(/read-only|permission|denied/);
+    });
+
+    it('workspace-write: the workspace is readable and writable', async () => {
+      const r = await runSandboxed(
+        `cat ${cwd}/seeded.txt && echo ok > ${cwd}/w.txt && cat ${cwd}/w.txt`,
+        cwd,
+        { ...unset, mode: 'workspace-write' },
+      );
+      expect(r.code).toBe(0);
+      expect(r.stdout).toContain('from outside');
+      expect(r.stdout).toContain('ok');
+    });
+
+    it('workspace-write: outside the workspace stays read-only', async () => {
+      const r = await runSandboxed('echo x > /etc/dc-should-not-exist', cwd, {
+        ...unset,
+        mode: 'workspace-write',
+      });
+      expect(r.code).not.toBe(0);
+    });
+
+    it('danger-full-access: no bwrap at all', async () => {
+      const wrapped = await wrapBashCommand({
+        userCommand: 'true',
+        cwd,
+        config: { ...unset, mode: 'danger-full-access' },
+      });
+      expect(wrapped.command).toBe('/bin/sh');
+    });
+
+    it('defaultMode applies when the config names no mode', async () => {
+      // A host passing workspace-write must get a sandbox, not the historical
+      // "off unless configured" behaviour.
+      const wrapped = await wrapBashCommand({
+        userCommand: 'true',
+        cwd,
+        config: unset,
+        defaultMode: 'workspace-write',
+      });
+      expect(wrapped.command).toBe('bwrap');
+
+      const r = await runSandboxed(`echo ok > ${cwd}/d.txt`, cwd, unset, 'workspace-write');
+      expect(r.code).toBe(0);
+    });
+
+    it('a library caller with no mode anywhere is still unsandboxed', async () => {
+      // Stated in wrapBashCommand's contract: an embedder must not become
+      // sandboxed by upgrading.
+      const wrapped = await wrapBashCommand({ userCommand: 'true', cwd, config: unset });
+      expect(wrapped.command).toBe('/bin/sh');
+    });
+  });
 });
diff --git a/packages/core/src/sandbox/profile.test.ts b/packages/core/src/sandbox/profile.test.ts
index 854ac60..306afe6 100644
--- a/packages/core/src/sandbox/profile.test.ts
+++ b/packages/core/src/sandbox/profile.test.ts
@@ -109,6 +109,25 @@ describe('buildLinuxBwrapArgs', () => {
     expect(args[idx + 2]).toBe('/my/project');
   });
 
+  it('does not bind cwd read-write under read-only', () => {
+    // bwrap applies binds in order and the last wins, so an unconditional
+    // `--bind cwd` at the end silently overrode the read-only bind that
+    // sandboxConfigForMode had asked for. The mode resolved correctly and the
+    // workspace was writable anyway.
+    const args = buildLinuxBwrapArgs(
+      { enabled: true, mode: 'read-only', filesystem: { allowRead: ['/my/project'] } },
+      '/my/project',
+    );
+    expect(args).not.toContain('--bind');
+    const last = args.lastIndexOf('/my/project');
+    expect(args[last - 2]).toBe('--ro-bind-try');
+  });
+
+  it('still binds cwd read-write under workspace-write', () => {
+    const args = buildLinuxBwrapArgs({ enabled: true, mode: 'workspace-write' }, '/my/project');
+    expect(args).toContain('--bind');
+  });
+
   it('unshares pid/ipc/uts', () => {
     const args = buildLinuxBwrapArgs({ enabled: true }, '/x');
     expect(args).toContain('--unshare-pid');
diff --git a/packages/core/src/sandbox/profile.ts b/packages/core/src/sandbox/profile.ts
index e0c665c..89d3a16 100644
--- a/packages/core/src/sandbox/profile.ts
+++ b/packages/core/src/sandbox/profile.ts
@@ -225,8 +225,23 @@ export function buildLinuxBwrapArgs(
   for (const p of fs.allowWrite ?? []) {
     args.push('--bind-try', p, p);
   }
-  // cwd is rw by default
-  args.push('--bind', cwd, cwd);
+  // cwd has to be visible or nothing works, but whether it is *writable* is the
+  // mode's decision — and this line used to make it unconditionally writable.
+  //
+  // bwrap applies binds in order and the last one wins, so under `read-only`
+  // the `--ro-bind-try` this loop already emitted for cwd (sandboxConfigForMode
+  // puts it in allowRead) was immediately overwritten by a read-write bind. The
+  // mode resolved correctly, the profile said the right thing, and a command
+  // could still write to the workspace.
+  //
+  // macOS never had this: buildMacOsProfile grants writes only from allowWrite,
+  // which read-only leaves empty. #226 verified the mode axis on macOS alone,
+  // and this is the half that was not looked at.
+  //
+  // An absent mode keeps the historical read-write bind, so a caller using the
+  // legacy `enabled: true` shape is unaffected.
+  if (config.mode === 'read-only') args.push('--ro-bind-try', cwd, cwd);
+  else args.push('--bind', cwd, cwd);
 
   // Network — three modes:
   //  1. allowedDomains: [] → no network at all